Phthalocyanine based inks with absorption maxima in the near infra-red and visible spectrum

ABSTRACT

The invention provides an ink composition comprising a charge generating material and a medium wherein the charge generating material absorbs radiation in both the near infra-red and visible regions of the electromagnetic spectrum. Also provided is an electrophotographic toner composition comprising a binder resin and the charge generating material. The compositions are useful in, for example, security marking wherein the composition is printed on an article or substrate. Methods of establishing the authenticity of an article or substrate are also provided which comprise detecting an absorption of infra-red radiation by the charge generating material.

FIELD OF THE INVENTION

This invention relates to charge generating materials and compositionscontaining them, for example inks and toners for printing andelectroreprographic use, to printing processes using such materials andcompositions on a range of substrates and articles, particularly for usein security marking or labelling and to uses of such materials andcompositions, e.g. in security applications. Such materials andcompositions are typically used in security printing applications forcounterfeit and fraud detection.

RELATED BACKAROUND ART

Inks are known which comprise substances acting as security markerswhich absorb little visible light but which are infrared absorbents.These marker substances may be included in inks in order to permit theeasy checking of matter printed with such inks, to assess theirauthenticity. The former is of significance if counterfeit goods areavailable, particularly if the safety of users is threatened, forexample in the case of medicines or safety equipment such as brakelinings. In any case, the reputation and business of the authenticsupplier will be threatened if counterfeit goods are available andprecautions may be required.

Some substances represent an additional component of the ink and add toits cost but still suffer disadvantages. In general they show someabsorption in the visible range which limits the concentration which canbe used without unacceptable discolouration of the ink from eitherwavelength shifts or dulling effects which limits the magnitude ofcharacteristic absorption which such inks can exhibit outside thevisible range. The problem may be increased if the ink includes blue orgreen colorants because the absorption band of such colorants is likelyto extend into the infrared region and to mask at least in part theabsorption of the marker substance.

We have found that such difficulties can be overcome by the use ofcharge generating materials which have both a strong absorption peak inthe infrared range and a significant absorption in the visible region ofthe electromagnetic spectrum sufficient to be an effective colorant. Theterm charge generating material used herein defines a material which iscapable of absorbing electromagnetic radiation in the visible region togenerate a cation-electron pair. Although such materials may typicallyhave application in electroreprographic processes to generate charge ona photoreceptor drum in an electroreprographic machine, it should beunderstood that in the present invention these materials are notemployed for their ability to generate charge. Instead, it has beenfound that such materials having absorptions in both the near infra-redand visible regions of the electromagnetic spectrum are useful, forexample, in inks for security marking.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention there is providedan ink composition comprising a charge generating material and a mediumwherein the charge generating material absorbs radiation in both thenear infra-red and visible regions of the electromagnetic spectrum.

The charge generating material preferably has two absorption maxima orpeaks: one in the near infra-red region and one in the visible region ofthe electromagnetic spectrum. Preferably, the charge generating materialhas an absorption maximum at a wavelength greater than 700 nm, morepreferably in the region from 700 to 1500 nm, most preferably from 700to 1000 nm, and an absorption maximum in the region from 400 nm to 700nm, more preferably from 500 to 700 nm, most preferably from 600 to 700nm. It is desirable that the two absorptions are quite separate,preferably that the separation is at least 50 nm, more preferably atleast 80 nm, most preferably at least 100 nm.

Preferably the height of the absorption peak in the near infrared rangebears a fixed ratio to the absorption peak in the visible range. Thisaids the authentication process when the present materials are used insecurity applications.

The strength of the absorption (i.e. as determined by the area under thepeak) in the visible region is preferably from 20 to 500% of thestrength of the absorption in the near infra-red region, more preferablyfrom 30 to 200%, most preferably 50 to 100% and especially 80 to 95%.

The peak in the infrared range is preferably sharp; at 80% of its heightjudged from the baseline it is preferably of bandwidth at most 150 nmand more preferably at most 100 nm.

It is also preferable that the charge generating material is stable andadequately lightfast. The quantity used of such charge generatingmaterials is preferably up to 10% by weight of the total composition,producing a very easily detectable peak in the infrared range. Thebenefit is particularly relevant in the case where the charge generatingmaterial is a green and especially a blue colorant.

The charge generating material may be the only colorant of inks ofcorresponding colour. Alternatively the charge generating material maybe used with one or more colorants of the same, similar or differentcolour; for example, a blue charge generating material can be used withanother blue colorant such as a dye or pigment, or, a blue chargegenerating material can be used to make green or brown inks whichadditionally contain respectively yellow or magenta colorants. Thecharge generating material may also be used in mixtures of colorant orfor adjusting or optimising the shades of dyes and/or pigments. Mixturesof two or more different charge generating materials according to theinvention may be used

The charge generating materials of the present invention act as securitymarkers and may be used in monochrome and/or multicolour printing.

The charge generating material is preferably selected from the followingclasses of compounds: phthalocyanines and their metal complexes,naphthalocyanines and their metal complexes, cyanines or polymethines,squaryliums, croconiums, iminiums, diimminiums, pyryliums, quinones, azodyes and their metal complexes. Each of the charge generating materialsmay be optionally substituted.

Preferably, the charge generating material is a phthalocyanine, and ischosen from compounds as shown in Formula (1) where M representshydrogen, a metal, metal hydroxide, metal oxide or alkoxide, metalhalide such as fluoride, chloride, bromide or iodide and k is theinverse of half the valency of M.

In a preferred subgroup of phthalocyanines, the peripheral carbon atoms1–16 in formula 1 are unsubstituted, i.e. they are bonded to hydrogen.Preferably, M is selected from H, GaOH, Ga(alkoxide), TiO and VO. WhenM=Ga(alkoxide) the alkoxide is preferably straight or branched chainC₁₋₈ alkoxide, more preferably OCH₃. Many such phthalocyanine compoundsexist in several different crystal forms or polymorphs, resulting frommolecular association within the crystal lattice. The properties ofthese compounds are influenced by this polymorphism; for example, thereis known to be a correlation between their electrical properties such asphotosensitivity and polymorphic form. Highly photosensitivephthalocyanines of interest in the present invention exhibit a commonmolecular overlapping pattern in their crystal structures. A result ofthis is an absorption spectrum which shows two peaks largely shiftedfrom that of a monomer spectrum; one is red-shifted and the other isblue-shifted. Especially preferred phthalocyanines are the polymorphtype X form of metal free (M=H, k=2) phthalocyanine, as described inU.S. Pat. No. 3,357,989 and by Sharp and Landon in J.Phys.Chem., (1968),72 (9), pp3230–3235, the polymorph type Y form and Phase I & 11 forms oftitanyloxy phthalocyanine (M=TiO, k=1), the polymorph Phase II form ofvanadyloxy phthalocyanine (M=VO, k=1), the polymorph Phase V form ofhydroxygallium phthalocyanine (M=GaOH, k 1) and methoxygalliumphthalocyanine (M=GaOMe, k=1) as described by Dalmon et.al. in J.Imaging Science & Technology, 1996, 40(3), p 249–253, and Oka and Okadain J. Imaging Science & Technology, 1993, 37(6), pp607–609.

The medium in the present invention may be a liquid medium or lowmelting point solid medium and may optionally contain one or moreadditives. Where the medium is a liquid, preferred media include water,a mixture of water and an organic solvent, or an organic solvent freefrom water. It is preferred that the organic solvent present in themixture of water and organic solvent is a water-miscible organic solventor mixture of such solvents. Preferred water-miscible organic solventsinclude C₁₋₆-alkanols, linear amides, ketones and ketone-alcohols,water-miscible ethers, diols, triols, mono-C₁₋₄-alkyl ethers of diols,preferably mono-C₁₋₄-alkyl ethers of diols having 2 to 12 carbon atoms,cyclic amides, cyclic esters, and sulphoxides. Examples of suitableliquid ink media comprising a mixture of water and one or more organicsolvents are described in U.S. Pat. No. 4,963,189, U.S. Pat. No.4,703,113, U.S. Pat. No. 4,626,284 and EP 4,251,50A. Preferably, thecharge generating material is insoluble in the medium. Where thematerial exists with a particular morphological form which provides therequired spectral properties, if the charge generating material issoluble in the medium, then this form may typically be lost upondissolution. The charge generating material may be present as asuspension, dispersion, colloidal suspension or otherwise.

Where no water is present the organic solvent may be selected from thosewell recited in the art, according to the standard categorization ofgravure inks, from the following classes of compound: aliphatichydrocarbons, aromatic hydrocarbons, esters, ketones and alcohols.Typical aliphatic hydrocarbon solvents include hexane, textile spirits,naphtha and mineral spirits well known in the trade; aromatichydrocarbon solvents typically comprise toluene or xylene; esters aretypically ethyl acetate, isopropyl acetate, n-propyl acetate and butylacetate; ketones are typically acetone and methyl ethyl ketone; alcoholsare selected from methanol, ethanol, propan-1-ol, propan-2-ol andbutanols, especially ethanol or propan-2-ol. Blends of these categoriesof solvents may also be used.

Where the medium is a low melting point solid it may be selected fromsolids whose melting point or T_(g) is less than 200° C., preferablyless than 150° C., more preferably less than 100° C., most preferablyless than 50° C. Such media include compounds which are usually solid orsemi-solid at ambient temperatures. Typical compounds include waxes,polymers, alkanes, alcohols and diols.

The medium may also comprise resins to impart adhesion and resistanceproperties to the printed ink film, and to fix the agents (especiallythe pigmentary materials) to the substrate surface. Such resins arepolymers which may be film formers or non-film formers. Resins may beselected from but are not limited to the following classes of compounds:acrylic, alkyd, cellulosic, chlorinated rubber, epoxy, nitrocellulose,phenolic, polyamide, polyester, polyketone, polystyrene, polyurethane,rosin-based, Saran, shellac and vinyl and products of the reaction ofsuch resins with oils such as linseed oil or tung oil. Optionally themedium may also comprise a plasticiser to achieve film formation.Typically a plasticiser will be selected from non-volatile, low molarmass compounds which are compatible with the main resins. Optionally oneor more additives may be present to modify the properties of thecomposition in some beneficial way, for example to prolong stability orto optimise the performance of the /dispersion when in contact with thesubstrate. Such additives may include one or more of the followingtypes: surfactants to modify surface tension; hydrotroping agents todisaggregate the system and prevent excess moisture loss; co-solvents toprevent excess moisture loss, for instance ink solvents such as InkSolvent 27/29, available from Shell Chemicals; acids or alkalis toachieve the optimum pH of the composition; buffers to maintain theoptimum pH range of the composition; polymers to act as viscositymodifiers; crystal poisoners to prevent crystallisation in dispersions;associative thickeners for dispersion stability; biocides where/dispersions are expected to have a finite shelf life; penetratingsolvents; metallic driers, typically soaps of cobalt, manganese andother metals formed with organic acids such as linoleic, naphthenic andoctanoic, to aid drying of the ink film by oxidation; waxes such aspolyethylene, Fischer Tropsch, Teflone™ or vegetable waxes; oils such aslinseed oil; white pigments to optimise the final shade of the printedink such as titanium dioxide or zinc dioxide; transparent white pigmentsor extenders such as alumina hydrate, magnesium carbonate, calciumcarbonate, precipitated barium sulphate, talc and clay; antioxidants toconfer stability against premature oxidation such as eugenol and ionol.

In a specific embodiment, the composition may be prepared by addition ofa charge generating material according to the invention in givenquantity to a liquid medium in which it is known to disperse, followedby stirring for such time until a homogeneous dispersion of known weightper unit volume is obtained, followed by the addition of such additivesas are necessary to maintain the stability and improve the properties ofthis dispersion. In yet another specific embodiment, where the chargegeneration material is a non-self dispersing pigment, it is necessary toadd the correct proportion of a dispersing agent that may be non-ionic,anionic or cationic, and this would normally be done during pigmentmilling although additional dispersing agent may be added to the finalcomposition for stability.

According to another aspect, the invention provides a toner compositionfor use in an electrophotographic process comprising a binder resin anda charge generating material wherein the charge generating materialabsorbs radiation in both the near infrared and visible regions of theelectromagnetic spectrum. The preferred features described aboverelating to the charge generating material are equally applicable to thetoner. The toner may be a conventionally produced toner wherein thecharge generating material is dispersible in the binder resin, and theseand any other components mentioned below are compounded and kneadedtogether. After cooling and pulverizing the resultant mixture, theparticles are classified. Alternatively, the toner may be a chemicallyproduced toner.

The binder resin of the toner may comprise one or more polymers,mixtures of polymers being appropriate where polymers of differentmolecular weight are used to control the molecular weight distributionand the melt rheology properties of the toner. Examples of suitablepolymers are styrene-acrylate copolymers, styrene-butadiene copolymers,polyesters and hydrocarbon resins.

The charge generating material may be the only colorant of toners ofcorresponding colour. Alternatively the charge generating material maybe used with one or more colorants of the same, similar or differentcolour; for example, a blue charge generating material can be used withanother blue colorant such as a dye or pigment, or, a blue chargegenerating material can be used to make green or brown toners whichadditionally contain respectively yellow or magenta colorants. Thecharge generating material may also be used in mixtures of colorant orfor adjusting or optimising the shades of dyes and/or pigments. Mixturesof two or more different charge generating materials may be used in thetoner.

The toner may incorporate a charge control agent to enhance uptake ofelectrical charge and a release agent, e.g. wax, to aid release from thefusion roller. Suitable charge control agents include metal complexes,such as complexes of Zn, Al, Fe or Cr, and polymeric materials, such asphenolic polymers. Suitable waxes include hydrocarbon waxes, such asparaffin, polyethylene or polypropylene waxes, waxes derived from carbonmonoxide and hydrogen, such as Fischer-Tropsch waxes, natural productwaxes, such as carnauba wax, and synthetic waxes, such as ester or amidewaxes. The toner may also contain surface additives, such as silica,titania, alumina or polymeric particles, to control flow, chargingperformance or transfer properties.

The invention also comprises the use of a charge generating material orcomposition comprising a charge generating material as described abovein security applications, for example as a security marker, preferablywhere the composition is applied by a variety of methods to articles orsubstrates. Examples of substrates are generally paper, including ragpaper, preferably currency grade paper, plastics-coated or laminatedpaper, and plastics such as bankcard-grade PVC, or plastic paper, e.g.nonwoven plastic paper. Examples of articles include documents,packaging or goods bearing a printed mark such as banknotes, banknotethread, currency, travellers' cheques, bonds, certificates, stamps,lottery tickets, ownership documents, passports, identity cards, creditcards, charge cards, access cards, smart cards, brand authenticationlabels and tags, and tamperproof labels.

According to a further feature of the present invention, there isprovided a method of establishing the authenticity of the article orsubstrate whereby the article or substrate is marked with a compositionaccording to the invention and a characteristic absorption of infraredradiation by the mark is detected, e.g. by standard spectroscopicmethods and optionally compared with a peak intensity of absorption ofradiation in the visible range. Compositions may for example be appliedby printing the composition onto the article or substrate. The method ofprinting is preferably selected from the following: offset lithographic,gravure, ink jet, intaglio, and letterpress. The composition of theinvention may be used in matrix or daisy-wheel printer inks andnon-impact printing methods.

EXAMPLES

The invention is further illustrated by reference to the followingexamples:

1. Preparation of Charge Generating Materials

1.1X-Form Metal-Free phthalocyanine (Formula 1, M=H, k=2)

This was prepared and characterised in accordance with literaturereferences US patent number U.S. Pat. No. 3,357,989 and Sharp and Landonin J.Phys.Chem., (1968), 72 (9), pp3230–3235.

1.2 Y Form and Phase I & 11 Forms of titanyloxy phthalocyanine (M=TiO,k=1)

These were prepared and characterised in accordance with literaturereferences Fujimati et al, J. Imaging Science & Technology, 1993, 37,13, Saito et. al., J. Phys. Chem., (1994), 98(11), 2726–8 and Okaet.al., Jpn. J. Appi. Phys., 1992, 31, 2181.

1.3 Polymorph Phase II form of vanadyloxy phthalocyanine (M=VO, k=1)

This was prepared and characterised in accordance with literaturereference Ziolo et.al., J. Chem. Soc., Dalton Trans., (1980), (11),2300–2.

1.4 Polymorph Phase V Form of hydroxygallium phthalocyanine (M=GaOH,k=1)

This was prepared and characterised in accordance with literaturereference Dalmon et.al. in J. Imaging Science & Technology, 1996, 40(3),p 249–253.

1.5 Methoxygallium phthalocyanine (M=GaOMe, k=1)

This was prepared and characterised in accordance with literaturereference Dalmon et.al. in J. Imaging Science & Technology, 1996, 40(3),p 249–253.

2. Security Printing

The following printing ink compositions were prepared using the X-formmetal free phthalocyanine as prepared above:

2.1 Intaglio printing ink formulation containing X-form phthalocyaninepigment Addition product of tung oil and maleic acid modified phenolic 35% resin in a high boiling mineral oil (PKWF 28/31) Long oil alkydresin 7.5% Alkylphenolic resin modified with raw tung oil in ink solvent 16% 27/29 (Shell Industrial Chemicals) Polyethylene wax 1.5% Calciumcarbonate 32.8%  X-form phthalocyanine pigment   2% Ink solvent 27/29(Shell Industrial Chemicals)   5% Cobalt octoate (11% metal) 0.1%Manganese octoate (10% metal) 0.1%

2.2 Lithographic printing ink formulation containing X-formphthalocyanine pigment Phenolic resin modified resin cooked with linseedoil 35%  Long oil alkyd resin 44.2%   Polyethylene wax 2% Ink solvent27/29 (Shell Industrial Chemicals) 2% Cooked linseed oil 5% X-formphthalocyanine pigment 5% Titanium dioxide 6% Cobalt octoate (10% metal)0.8%  

2.3 Letterpress ink formulation containing X-form phthalocyanine pigmentPhenolic resin modified resin cooked with linseed oil 31%  Long oilalkyd resin 42.2%   Polyethylene wax 2% Ink solvent 27/29 ShellIndustrial Chemicals 2% Cooked linseed oil 5% X-form phthalocyaninepigment 5% Titanium dioxide 6% Silicium dioxide (AEROSIL 200 - Degussa -Huels AG) 6% Cobalt octoate (10% metal) 0.8%  

2.4 Gravure ink formulation containing X-form phthalocyanine pigmentEthanol 20% Ethyl acetate 20% Dicyclohexylphthalate (Unimoll 66,supplied by Bayer)  4% Fumaric acid modified rosin (Rokramar 7200,supplied by  4% Robert Kraemer GmbH & Co) Polyvinylbutyral resin(Pioloform BN 18, supplied by 10% Wacker-Chemie) X-form phthalocyaninepigment  3% Ethanol 12% Ethyl acetate 27%

The printing inks were prepared by dispersing the compounds in a beadmill. In the gravure composition a proportion of the ethanol (20%) andthe ethylacetate (20%) was added during the dispersion process, theremaining amount being added after dispersion. Prints were made usingstandard industry printing apparatus and analysed using a standardUV-VIS-NIR spectrometer.

1. In a security method comprising application of an ink composition,the improvement wherein the composition comprises a charge generatingmaterial and a medium wherein the charge generating material has anabsorption maximum in each of the near infra-red region from 700 to 1500nm and visible region from 400 to 700 nm of the electromagnetic spectrumand is a compound selected from the polymorph type X form of metal freephthalocyanine, the polymorph type Y form and Phase I & II forms oftitanyloxy phthalocyanine, the polymorph Phase II form of vanadyloxyphthalocyanine, and the polymorph Phase V forms of hydroxygalliumphthalocyanine and methoxygallium phthalocyanine and wherein the chargegenerating material acts as a colorant of the composition.
 2. A methodaccording to claim 1 wherein the charge generating material is theX-form of metal free phthalocyanine.
 3. A method according to claim 1wherein the charge generating material has an absorption maximum in theregion from 500 to 700 nm.
 4. A method according to claim 3 wherein thecharge generating material has an absorption maximum in the region from600 to 700 nm.
 5. A method according to claim 1 wherein the twoabsorption maxima are separated by at least 50 nm.
 6. A method accordingto claim 1 wherein the strength of the absorption in the visible regionis between 20 and 500% of the strength of the absorption in the nearinfra-red region.
 7. A method according to claim 1 wherein the peak inthe near infra-red range is, at 80% of its height judged from thebaseline, of bandwidth at most 150 nm.
 8. A method according to claim 1wherein the charge generating material is the only colorant of the ink.9. A method according to claim 1 wherein the charge generating materialis used with one or more additional colorants of the same, similar ordifferent colour.
 10. A method according to claim 1 the ink compositioncomprising a mixture of two or more different charge generatingmaterials.
 11. A method according to claim 1 wherein the chargegenerating material is insoluble in the medium.
 12. A method accordingto claim 1 wherein the composition is applied by a printing method to anarticle or substrate.
 13. A method according to claim 12 wherein theprinting method is selected from the following: offset, gravure, inkjet, intaglio, electrophotography.
 14. A method according to claim 1wherein the security application is a method of establishing theauthenticity of an article or substrate which has been marked with thesaid ink composition comprising detecting a characteristic absorption ofinfrared radiation by the mark.
 15. A method according to claim 14 themethod further comprising comparing the characteristic absorption ofinfrared radiation with an absorption of radiation in the visible range.16. A method according to claim 15 wherein the height of the absorptionpeak in the near infrared range is compared with the height of theabsorption peak in the visible range.
 17. In a security application ofan ink composition, the improvement wherein the composition comprises acharge generating material and a medium wherein the charge generatingmaterial has an absorption maximum in each of the near infra-red regionfrom 700 to 1500 nm and visible region from 400 to 700 nm of theelectromagnetic spectrum, wherein the charge generating material is theonly colorant of the ink.
 18. A method of establishing the authenticityof an article or substrate which has been marked with an inkcomposition, the ink composition comprising a charge generating materialand a medium wherein the charge generating material has an absorptionmaximum in each of the near infra-red region from 700 to 1500 nm andvisible region from 400 to 700 nm of the electromagnetic spectrum, themethod comprising detecting a characteristic absorption of near infraredradiation by the mark and further comprising comparing thecharacteristic absorption of near infrared radiation with an absorptionof radiation in the visible range, the charge generating material actingas a colorant of the ink composition.
 19. A method according to claim 18wherein the height of the absorption peak in the near infrared range iscompared with the height of the absorption peak in the visible range.20. A method according to claim 5 wherein the two absorption maxima areseparated by at least 80 nm.
 21. A method according to claim 5 whereinthe two absorption maxima are separated by at least 100 nm.
 22. A methodaccording to claim 6 wherein the strength of the absorption in thevisible range is between 30 to 200% of the strength of the absorption inthe near infrared region.
 23. A method according to claim 6 wherein thestrength of the absorption in the visible range is between 50 to 100% ofthe strength of the absorption in the near infrared region.
 24. A methodaccording to claim 6 wherein the strength of the absorption in thevisible range is between 80 to 90% of the strength of the absorption inthe near infrared region.
 25. A method according to claim 7 wherein thepeck in the near infrared range is, at least 80% of its height judgedfrom the baseline, of bandwidth at most 100 nm.